Should age automatically trigger amniocentesis?

Posted: Tuesday, November 11, 2003

Lauran NeergaardAP Medical Writer

WASHINGTON - Is simply being 35 or older enough reason for a pregnant woman to undergo amniocentesis to check for Down syndrome?

The older mothers-to-be are, the higher their risk of having babies with this fairly common birth defect. But nothing suddenly changes at the 35th birthday, and a 20-something can have a baby with Down syndrome, too.

Now growing evidence suggests that offering blood tests to screen every pregnant woman for the risk may be more effective than using today's standard age cutoffs for amniocentesis - decreasing unnecessary amnios by culling out which women most need the invasive test, which is more accurate but occasionally causes miscarriage.

"Twenty years ago, the only way to identify women at risk was to offer amnio to those older than 35. That makes no sense any longer," says Dr. Rebecca Smith-Bindman of the University of California, San Francisco, who co-authored a large new study that supports screening-based instead of age-based decisions.

On The Net:

California testing program: www.dhs.ca.gov/pcfh/gdb/html/PS/PS.htm

It's a policy that California has adopted, with a program that provides free amniocentesis for women whose tests determine they're at high risk.

Whether to have an amnio is an agonizing decision that more Americans are making as they increasingly postpone childbearing into their 30s and even 40s. But women have a 1 percent chance of suffering a miscarriage from amniocentesis, the use of a needle to draw fluid from the amniotic sac, or from a similar invasive test called chorionic villus sampling. While it's a small risk, some women won't chance a test that could cost them a healthy baby.

While risk increases with each year of maternal age - from one in 1,200 at age 25 to about one in 300 at 35 - most Down syndrome babies are born to women under 35 simply because younger women have more babies.

Hence, women under 35 are supposed to be offered certain blood tests that can identify about three-quarters of Down syndrome cases by measuring different chemicals in the mother's blood. Missed cases aside, blood tests also can cause false alarms, so a positive blood test requires a definitive amnio to double-check.

The new study suggests that blood tests, or specialized ultrasound exams that also can spot defects, should be the amnio deciding factor even for older moms.

Smith-Bindman and British colleagues examined 6 million births over the last decade in England and Wales, where different regions use either age or blood/ultrasound screening to determine who needs an amnio.

In areas where women got amnios based on screening, 50 percent more Down syndrome cases were diagnosed prenatally, with half as many invasive tests performed, compared with areas where age determined testing, concludes the study in last month's American Journal of Obstetrics and Gynecology.